Jacques Loeb

“ The nature of life and of death are questions which occupy the interest of the layman to a greater extent than possibly any other purely theoretical problem; and we can well understand that humanity did not wait for experimental biology to furnish an answer. The answer assumed the anthropomorphic form characteristic of all explanations of nature in the prescientific period. Life was assumed to begin with the entrance of a "life principle" into the body; that individual life begins with the egg was of course unknown to primitive or pre-scientific man. ”

“ Our wishes and hopes, disappointments and sufferings have their source in instincts which are comparable to the light instinct of the heliotropic animals. The need of and the struggle for food, the sexual instinct with its poetry and its chain of consequences, the maternal instincts with the felicity and the suffering caused by them, the instinct of workmanship and some other instincts are the roots from which our inner life develops. ”

“ That in the case of our inner life a physico-chemical explanation is not beyond the realm of possibility is proved by the fact that it is already possible for us to explain cases of simple manifestations of animal instinct and will on a physico-chemical basis; namely, the phenomena which I have discussed in former papers under the name of animal tropisms. As the most simple example we may mention the tendency of certain animals to fly or creep to the light. We are dealing in this case with the manifestation of an instinct or impulse which the animals can not resist. ”

“ Scientifically, however, individual life begins (in the case of the sea-urchin and possibly in general) with the acceleration of the rate of oxidation in the egg, and this acceleration begins after the destruction of its cortical layer. Life of warm blooded animals—man included—ends with the cessation of oxidation in the body. As soon as oxidations have ceased for some time the surface films of the cells, if they contain enough water and if the temperature is sufficiently high, become permeable for bacteria, and the body is destroyed by microorganisms. ”

“ The specific substances which accelerate the oxidations at body temperature sufficiently to allow the maintenance of life are the so-called ferments of oxidation.
The work of Lavoisier and Laplace not only marks the beginning of scientific biology, it also touches the core of the problem of life; for it seems that oxidations form a part, if not the basis, of all life phenomena in higher organisms. ”

“ In these animals the sex is predetermined in the egg and not in the spermatozoon. It is of interest that, according to Guyer, in the human being two kinds of spermatozoa exist and only one kind of eggs; in man, therefore, sex is determined by the spermatozoon.
How is sex determination accomplished? Let us take the case which according to Wilson is true for many insects and according to Guyer for human beings, namely, that there are two kinds of spermatozoa and one kind of egg. According to Wilson all unfertilized eggs contain in this case one so-called sex chromosome, the X-chromosome. ”

“ It is seemingly often taken for granted by laymen that "truth" in biology or science in general is of the same order as "truth" in certain of the mental sciences; that is to say, that everything rests on argument or rhetoric and that what is regarded as true to-day may be expected with some probability to be considered untrue to-morrow. It happens in science, especially in the descriptive sciences like paleontology or zoology, that hypotheses are forwarded, discussed and then abandoned. ”

“ We know that every living being is able to transform food-stuffs into living matter; and we also know that not only the compounds which are formed in the animal body can be produced artificially, but that chemical reactions which take place in living organisms can also be repeated at the same rate and temperature in the laboratory. ”

“ The hereditary transmission of the black color through the male animal must occur by substances carried in the chromosome which determine the formation of tyrosin or tyrosinase or of both. We may, therefore, say that the solution of the riddle of heredity has succeeded to the extent that all further development will take place purely in cytological and physico-chemical terms. ”

“ We are active, because we are compelled to be so by processes in our central nervous system; and as long as human beings are not economic slaves the instinct of successful work or of workmanship determines the direction of their action. ”

“ If a color-blind individual is mated with an individual with normal color vision the heredity of color blindness in the next two generations corresponds quantitatively with what we must expect on the assumption that the chemical substances determining color vision are contained in the sex-chromosomes. In the color-blind individual something is lacking which can be found in the individual with normal color perception. The factor for color vision is obviously transmitted through the sex-chromosome. ”

“ It is, therefore, no exaggeration to state that the number of species existing to-day is only an infinitely small fraction of those which can and possibly occasionally do originate, but which escape our notice because they can not live and reproduce. Only that limited fraction of species can exist which possesses no coarse disharmonies in its automatic mechanism of preservation and reproduction. Disharmonies and faulty attempts in nature are the rule, the harmonically developed systems the rare exception. ”

“ The question arises as to how the destruction of the cortical layer can cause the beginning of the development of the egg. This question leads us to the process of oxidation. Years ago I had found that the fertilized sea-urchin egg can only develop in the presence of free oxygen; if the oxygen is completely withdrawn the development stops, but begins again promptly as soon as oxygen is again admitted. From this and similar experiments I concluded that the spermatozoon causes the development by accelerating the oxidations in the egg. ”

“ The vitalists deny the possibility of a complete explanation of life in terms of physics and chemistry. The mechanists proceed as though a complete and unequivocal physico-chemical analysis of life were the attainable goal of biology. ”

“ That a part is so constructed that it serves the "whole" is only an unclear expression for the fact that a species is only able to live—or to use Roux's expression—is only durable, if it is provided with the automatic mechanism for self-preservation and reproduction. If, for instance, warm-blooded animals should originate without a circulation they could not remain alive, and this is the reason why we never find such forms. The phenomena of "adaptation" cause only apparent difficulties since we rarely or never become aware of the numerous faultily constructed organisms which appear in nature. ”

“ While until twelve years ago the field of heredity was the stamping ground for the rhetorician and metaphysician it is to-day perhaps the most exact and rationalistic part of biology, where facts can not only be predicted qualitatively, but also quantitatively. ”

“ As long as a life phenomenon has not yet found a physico-chemical explanation it usually appears inexplicable. If the veil is once lifted we are always surprised that we did not guess from the first what was behind it. ”

“ The mother loves and cares for her children not because metaphysicians had the idea that this was desirable, but because the instinct of taking care of the young is inherited just as distinctly as the morphological characters of the female body. ”

“ It is not possible to prove in a short address that all life phenomena will yield to a physico-chemical analysis. We have selected only the phenomena of fertilization and heredity, since these phenomena are specific for living organisms and without analogues in inanimate nature ”

“ The behavior of the animals agrees with this assumption. If the light is sufficiently strong the animals go in an almost straight line to the source of light; if the intensity of light (or the mass of photosensitive substances on the surface of the animal) is small the animals go in irregular lines, but at last they also land at the source of light, since the directing force is not entirely abolished. It will, however, be necessary to ascertain by direct measurements to what extent these phenomena in animals are the expression of Bunsen-Eoscoe's law. ”

“ We all, however, desire to know how life originates and what death is, since our ethics must be influenced to a large extent through the answer to this question. We are not yet able to give an answer to the question as to how life originated on the earth. ”

“ The gap in our knowledge which we feel most keenly is the fact that the chemical character of the catalyzers (the enzymes or ferments) is still unknown. Nothing indicates, however, at present that the artificial production of living matter is beyond the possibilities of science. ”

“ We seek and enjoy the fellowship of human beings because hereditary conditions compel us to do so. We struggle for justice and truth since we are instinctively compelled to see our fellow beings happy. Economic, social and political conditions or ignorance and superstition may warp and inhibit the inherited instincts and thus create a civilization with a faulty or low development of ethics. Individual mutants may arise in which one or the other desirable instinct is lost, just as individual mutants without pigment may arise in animals ”

“ Morgan has observed a mutation with white eyes, which occurs in the male. When he crossed a white-eyed with a red-eyed female all flies of the first generation were red-eyed; since all flies had the factor for pigment in their sex-cells; in the second generation all females and exactly one half of the males had red eyes, the other half of the males, however, white eyes, as the theory demands. ”

“ It should also be stated that whenever a vitalist desires to make a contribution to science which is more substantial and lasting than mere argument or metaphor, he forgets or lays aside his vitalism and proceeds on the premises and methods of the mechanist. ”

“ If we cross two forms which differ in only one character every hybrid resulting from this union forms two kinds of sex-cells in equal numbers; two kinds of eggs if it is a female, two kinds of spermatozoa if it is a male. ”

“ Consequently the development of energy in the symmetrical muscles on both sides of the body becomes unequal. As soon as the difference in the masses of the photochemical reaction products on both sides of the animal reaches a certain value the animal, as soon as it moves, is automatically forced to turn towards one side. ”

“ The products formed in this process influence the contraction of the muscles—mostly indirectly, through the central nervous system. If the animal is illuminated on one side only the mass of photochemical reaction products formed on that side in the unit of time is greater than on the opposite side. ”